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Martin Osborne
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Homework Statement
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Hi, I have a problem I have been trying to do for a few days and I am not getting it. Any hints would be greatly appreciated. The question is from "The physics of Quantum Mechanics" by Binney and Skinner.
The question states:
Let ##\psi##(x) be a properly normalised wavefunction and Q an operator on wavefunctions. Let {qr} be the spectrum of Q and let {Ur(x)} be the corresponding correctly normalised eigenfunctions. Write down an expression for the probability that a measure of Q will yeild the value qr.
Show that ##\Sigma_r P(q_r |\psi) = 1##.
Show further that the expectation of Q is ## \langle Q \rangle = \int _{-\infty} ^\infty \psi^* Q \psi dx## .
Homework Equations
and attempt[/B]So for the first part, the probability amplitude of measuring qr given the system is in the state ## |\psi\rangle ## is given by ## \langle q _ r | \psi \rangle = \int _{-\infty} ^\infty u_r^*(x) \psi(x) dx## .
and taking the mod squared of this gives the probability the question is asking for.
The next part says that summing these probabilities over all r = 1? I understand what this means (probability of finding a value of q within the spectrum given = 1), but don't know how to show this.
As for the last part, the expectation value is the sum of the probabilities of getting each value of q multiplied by the value qr, so $$ \langle Q \rangle = \Sigma _ r q_r | \int _ {-\infty}^\infty u_r^*(x) \psi(x) dx |^2 $$Cant get any further...
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